Composite materials are today widely used for the manufacture of parts in many industrial fields, such as the aeronautical field, including for structural parts, that is to say parts that have to withstand significant stresses during their use.
A composite material generally consists of a fibrous reinforcement and a resin, the combination of which makes it possible to obtain a complex whose properties surpass those of the starting materials. Thus, composite material parts have many advantages linked in particular to their properties of strength, lightness and ease of forming.
The most commonly used method for producing a thermosetting composite material part consists in producing a structure by stacking reinforcing layers or plies of fibers preimpregnated with a thermosetting resin, referred to as a preimpregnated fibrous preform, with features and according to a shape adapted to that of the final composite material part to be produced, then in curing the resin, by polymerization, under a vacuum cover, in an autoclave or an oven. The part obtained has, throughout its thickness, the same degree of polymerization.
Another method also widely used for producing such a part essentially consists in diffusing thermosetting resin uniformly, by injection or infusion, into the fibrous preform, this time that is initially dry, having the shape of the part to be produced. After evacuation of the gases and distribution of the resin, the latter is cured, by polymerization, in a mold or a vacuum cover to give the desired composite material part. This method is RTM (Resin Transfer Molding) or LRI (Liquid Resin Infusion). In the same way as for the previous method, the part obtained has, throughout its thickness, the same degree of polymerization.
Certain aircraft structural parts are produced from individual parts, produced by one of the methods mentioned above, and joined to one another by bonding. For example, the fuselage or wing panels consist of an assembly of a skin and stiffeners.
Three bonding assembly methods are used in aeronautics:                the co-curing bonding method: it consists of the assembling by polymerization of two unpolymerized parts, the joint between the two parts being made with or without adhesive film;        the co-bonding method: it consists of the assembling by polymerization of one polymerized part and of one unpolymerized part, the joint between the two parts being made by an adhesive film;        the secondary-bonding method: it consists of the assembling by bonding of two polymerized parts, the joint between the two parts being made by an adhesive film.        
The co-curing bonding method has the advantage of not requiring sanding and cleaning surface preparation of the parts before assembly. It furthermore gives the final assembly a good mechanical strength.
The co-bonding and secondary-bonding methods have the advantage, unlike the co-curing bonding method, of simplifying the manufacturing, storing, handling and nondestructive testing operations of the individual parts polymerized before bonding. They also have a good mechanical strength of the final assembly.
Although satisfactory, these existing methods each have numerous drawbacks, among other things:                for the co-bonding and secondary-bonding methods:                    lack of a reliability of the bonded joint, i.e. a possible appearance of bonding having a low, or even very low mechanical strength in the joint, also referred to as a kissing bond;            inability to detect these kissing bonds by nondestructive testing and therefore an inability to detect the parts having a low mechanical strength in the bonded joint;            need for surface preparation of the parts;            recourse to fastenings to eliminate the risk of delamination at the bonding interface;                        for the co-curing bonding method:                    complex use of the equipment and in the handling of the unpolymerized parts and consequently a high cost for assembling two parts;            storage of the unpolymerized parts under specific conditions (generally at −18° C.) before bonding;            carrying out nondestructive testing operations after assembly and polymerization, and consequently a high complexity and a high cost for testing the assembly.                        
Consequently, there is a need to provide a solution that makes it possible to produce bonding between two parts having a good mechanical strength, and a reliability of the bonded joint similar to that obtained with the co-curing bonding method, while having a greater ease of use in order to reduce costs.